JEE Main 2025 Apr 8 Shift 2, Chemistry Q4: Preparation of Alkyl Halides (Halo Alkanes) & Aryl Halides (Halo Arenes)

Step 1: Bromination

Ethylbenzene reacts with $\mathrm{B}{\mathrm{r}}_2/Fe$ via electrophilic aromatic substitution (EAS). Since the ethyl group is an ortho/para-directing group, the major product is para-bromoethylbenzene.

$\boxed{\mathrm{C_6H_5CH_2CH_3} \xrightarrow{\mathrm{Br_2/Fe}} p\text{-}\mathrm{BrC_6H_4CH_2CH_3}}$

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Step 2: Benzylic Chlorination

Treatment with $\mathrm{C}{\mathrm{l}}_2$ under heat $(\Delta )$ proceeds via a free-radical substitution at the benzylic position.

$\boxed{ p\text{-}\mathrm{BrC_6H_4CH_2CH_3} \xrightarrow{\mathrm{Cl_2},\,\Delta} p\text{-}\mathrm{BrC_6H_4CHClCH_3} }$

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Step 3: Dehydrohalogenation

Alcoholic KOH causes E2 dehydrohalogenation, eliminating HCl to form the vinyl group $(-\mathrm{CH}=\mathrm{C}{\mathrm{H}}_2)$.

$\boxed{ p\text{-}\mathrm{BrC_6H_4CHClCH_3} \xrightarrow{\mathrm{alc.\ KOH}} p\text{-}\mathrm{BrC_6H_4CH{=}CH_2} }$

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Conclusion

The overall sequence converts ethylbenzene into p-bromostyrene.

$\boxed{\text{Required reagent sequence: Option (1)}}$